Method and apparatus for allocating demodulation reference signals

ABSTRACT

Provided are a method and apparatus for allocating DeModulation Reference Signals (DMRSs). The method includes generating DMRSs, and allocating the DMRSs at consecutive subcarrier positions with respect to all the transmit (TX) antennas of each User Equipment (UE) and allocating the DMRSs at different subcarrier positions with respect to each TX antenna of the UE.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean PatentApplication No. 10-2008-0131749, filed on Dec. 22, 2008, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The following disclosure relates to data transmission in a MultipleInput Multiple Output (MIMO) Single-Carrier Frequency Division MultipleAccess (SC-FDMA) system, and in particular, to allocation ofDeModulation Reference Signals (DMRSs) for DMRS transmission in theuplink transmission of a MIMO SC-FDMA system.

BACKGROUND

Wireless communication technologies are developing rapidly, andextensive research is being conducted particularly on methods fortransmitting a large amount of data at a high rate.

For high-rate data transmission, SC-FDMA has been proposed as a radioaccess scheme in 3rd Generation Partnership Protocol-Long Term Evolution(3GPP-LTE) uplink transmission.

In the 3GPP LTE, a basic uplink transmission scheme providesorthogonality for transmit (TX) signals between uplink users and it isbased on SC-FDMA transmission with a low Peak-to-Average Power Ratio(PAPR). Also, in order to secure a low PAPR, allocation of a frequencyband constituting one SC-FDMA symbol uses localized transmission.

FIG. 1 is a diagram illustrating allocation of DMRSs for the uplink of aSingle Input system (i.e., a Single Input Single Output system or aSingle Input Multiple Output system).

Uplink SC-FDMA traffic data is transmitted over a Physical Uplink SharedCHannel (PUSCH). DMRSs are transmitted using a Frequency DivisionMultiplexing (FDM) scheme of dividing User Equipments (UEs) in asubcarrier band on a subcarrier group-by-subcarrier group basis and aCode Division Multiplexing (CDM) scheme of allocating the samesubcarrier resource in an overlapping manner and dividing UEs by codes.

As illustrated in FIG. 1, in the uplink of a Single Input Single Outputsystem or a Single Input Multiple Output system, DMRSs are divided byFDM between UEs. Herein, the DMRSs of each UE are arranged consecutivelyin a subcarrier group.

FIG. 2 is a diagram illustrating allocation of DMRSs for the uplink of aMIMO system.

As illustrated in FIG. 2, DMRSs are divided by FDM between UEs and theyare divided by CDM between antennas of each UE. The DMRS M-K-I is the1^(th) DMRS allocated to the K^(th) antenna of the M^(th) UE.

The DMRSs of each antenna in each UE are allocated to the samesubcarrier for the same SC-FDMA symbol and are divided by CDM.Therefore, a channel estimation process in a receiver requires anoperation of separating DMRSs that are divided and transmitted by CDM.

This operation is performed on the DMRSs of each TX antenna that arereceived by a receiver. The operation, however, increases the complexityof the channel estimation and causes an error in the channel estimation,thus leading to the performance degradation.

SUMMARY

In one general aspect of the present invention, a method for allocatingDMRSs includes: generating DMRSs; and allocating the DMRSs atconsecutive subcarrier positions with respect to all the transmit TXantennas of each UE and allocating the DMRSs at different subcarrierpositions with respect to each TX antenna of the UE.

The generating of DMRSs may include cyclically shifting a base sequenceas many as the number of subcarriers for each TX antenna.

The DMRSs may be allocated to the respective TX antennas sequentiallyone by one.

The DMRSs may be allocate to the respective TX antennas one by one in asubcarrier group band where as many DMRSs as the number of the TXantennas of the UE are consecutively allocated.

In another general aspect, an apparatus for allocating DMRSs includes: amultiplexer multiplexing DMRSs by frequency division; and a subcarrierresource mapper allocating the multiplexed DMRSs with respect to TXantennas of each UE, wherein the subcarrier resource mapper allocatesthe DMRSs at different subcarrier positions with respect to each TXantenna of the UE and allocates the DMRSs at consecutive subcarrierpositions with respect to all the TX antennas of the UE.

The DMRSs may be generated by cyclically shifting a base sequence asmany as the number of subcarriers for each TX antenna.

The subcarrier resource mapper may allocate the DMRSs to the respectiveTX antennas one by one in a subcarrier group band where as many DMRSs asthe number of the TX antennas of the UE are consecutively allocated.

The subcarrier resource mapper may allocate the DMRSs to the respectiveTX antennas sequentially one by one.

Other features and aspects will be apparent from the following detaileddescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating allocation of DMRSs for the uplink of aSingle Input system.

FIG. 2 is a diagram illustrating allocation of DMRSs for the uplink of aMIMO system.

FIG. 3 is a diagram illustrating a TX frame structure of traffic data.

FIG. 4A and FIG. 4B are block diagrams of an uplink MIMO SC-FDMAtransmitter unit.

FIG. 5 is a diagram illustrating allocation of DMRSs according to anexemplary embodiment of the present invention.

FIG. 6 is a diagram illustrating allocation of DMRSs according toanother exemplary embodiment.

FIG. 7 is a diagram illustrating allocation of DMRSs according toanother exemplary embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, exemplary embodiments will be described in detail withreference to the accompanying drawings. Throughout the drawings and thedetailed description, unless otherwise described, the same drawingreference numerals will be understood to refer to the same elements,features, and structures. The relative size and depiction of theseelements may be exaggerated for clarity, illustration, and convenience.The following detailed description is provided to assist the reader ingaining a comprehensive understanding of the methods, apparatuses,and/or systems described herein. Accordingly, various changes,modifications, and equivalents of the methods, apparatuses, and/ofsystems described herein will be suggested to those of ordinary skill inthe art. Also, descriptions of well-known functions and constructionsmay be omitted for increased clarity and conciseness.

The exemplary embodiments relate to allocation of DMRSs to TX antennasof each UE in order to apply a SC-FDMA system for 3GPP LTE uplink datatransmission to a MIMO system.

A 3GPP LTE uplink SC-FDMA system uses an FDM scheme of providingorthogonality by dividing subcarrier allocation bands in a frequencyband for allocation of DMRSs between UEs and a CDM scheme of providingorthogonality by dividing codes in the same time/frequency bands.

When such a conventional DMRS allocation method is applied to a MIMOSC-FDMA system, each UE divides DMRSs by CDM between its TX antennas inorder to transmit the DMRSs through a plurality of the TX antennas.However, the CDM-based DMRS allocation between the TX antennas of eachUE increases the complexity of a channel estimator, which performschannel estimation in a data demodulator of a Base Station (BS)receiver, and also increases a channel estimation error, as describedabove.

Therefore, the exemplary embodiments provide a method for dividingDMRSs, which are allocated to and transmitted through TX antennas ofeach UE, by FDM in a system where each UE transmits signals through aplurality of TX antennas by MIMO SC-FDMA.

Herein, DMRSs are divided by FDM between UEs on a subcarrier groupbasis, and DMRSs are allocated by FDM between TX antennas of each UE.

Hereinafter, the exemplary embodiments will be described in detail withreference to the accompanying drawings.

FIG. 3 is a diagram illustrating a TX frame structure of traffic data.

Referring to FIG. 3, a TX frame of SC-FDMA traffic data in the 3GPP LTEis configured to include radio frames, subframes, slots, and SC-FDMAsymbols.

Each radio frame has a time length of 10 ms and includes 10 subframes.Thus, as illustrated in FIG. 3, each subframe has a time length of 1 msand includes 2 slots.

Each slot includes 7 SC-FDMA symbols. Each SC-FDMA symbol has a CyclicPrefix (CP).

Uplink SC-FDMA traffic data is transmitted over a PUSCH. Herein, the0^(th), 1^(st), 2^(nd), 4^(th), 5 and 6^(th) SC-FDMA symbols among atotal of 7 SC-FDMA symbols are transmitted by a TX slot. Traffic data isencoded, modulated and transmitted by the 0^(th), 1^(st), 2^(nd),4^(th), 5^(th) and 6^(th) SC-FDMA symbols, and a DMRS is transmitted bythe 3^(rd) SC-FDMA symbol.

The DMRS is used for channel estimation and Signal-to-Noise Ratio (SNR)estimation in a receiver, and it is used to demodulate/decode the datain a signal transmitted over the PUSCH.

FIG. 4A and FIG. 4B are block diagrams of an uplink MIMO SC-FDMAtransmitter unit. Block diagrams depicted herein are not separateembodiments regarding to the uplink MIMO SC-FDMA transmitter unit, butshow one embodiment as to the uplink MIMO SC-FDMA transmitter unit. Indetail, a few signals transmitted from a layer mapper 411 of FIG. 4A aresent to a layer interleaver 413 of FIG. 4B. Also, the same terms ofcomponents are used throughout FIGS. 4A and 4B to designate the same orsimilar function.

Referring to FIG. 4A and FIG. 4B, traffic data received from an upperlayer is processed through a channel encoder 401, a rate matcher 403, achannel interleaver 405, a bit scrambler 407 and a symbol constellationmapper 409. Next, it is processed through a layer mapper 411, a layerinterleaver 413 and a precoder 415. Thereafter, the traffic data isprocessed through a transform precoder 417 according to each antennapath.

Control data is processed through a control data encoder and a modulatorof a control data generator 419, and a DMRS is generated by a DMRSgenerator 421.

The output of the transform precoder 417 for the traffic data accordingto each antennal path, the output signal of the modulator of the controldata generator 419 for the control data, and the DMRS generated by theDMRS generator 421 are selected by a multiplexer (MUX) 423 according tothe time and the TX mode. Next, it is allocated by a subcarrier resourcemapper 425 to a subcarrier of a frequency band. Then, it is transformedusing Inverse Fast Fourier Transform (IFFT) by an IFFT processor 427.Thereafter, a CP is inserted by a CP inserter 429. Then, it istransmitted through each antenna.

Herein, a DMRS r_(u,v) ^((α))(n) generated by the DMRS generator 421 isgenerated by a cyclic shift a of a base sequence r _(u,v)(n) as Equation(1) below.

r _(u,v) ^((α))(n)=e ^(jon) r _(u,v)(n), 0≦n≦M _(sc) ^(RS)   (1)

Herein, M_(sc) ^(RS) is the number of subcarriers for transmission ofDMRSs in each SC-FDMA symbol.

Thus, a plurality of DMRS sequences are generated by the cyclic shift a.

The base sequence r _(u,v)(n) is determined according to a group numberu ∈ {0, 1, . . . , 29} and a base sequence number v in the group.

Herein, if the number of TX subcarriers is equal to or smaller than 60,v=0; and if not, v=0, 1.

The base sequence r _(u,v)(n) may be generated by the q^(th) rootZadoff-Chu sequence as Equation (2) below.

r _(u,v)(n)=x _(q)(n mod N _(ZC) ^(RS)), 0≦n≦M _(sc) ^(RS)   (2)

Herein, the q^(th) root Zadoff-Chu sequence is expressed as Equation (3)below.

$\begin{matrix}{{{x_{q}(m)} = ^{{- j}\frac{\pi \; {{qm}{({m - 1})}}}{N\frac{RS}{ZC}}}},{0 \leq m \leq {N_{ZC}^{RS} - 1}}} & (3)\end{matrix}$

Herein, q=└ q+½┘+v·(−1)^(└2 q┘) and q=N_(ZC) ^(RS)·(u+1)/31. Also, thelength N_(ZC) ^(RS) of the Zadoff-Chu sequence is the largest primenumber satisfying N_(ZC) ^(RS)<M_(sc) ^(RS)

Also, the base sequence r _(u,v)(n) may be generated as Equation (4)below.

r _(u,v)(n)=e ^(jφ(n)π/4), 0≦n≦M _(sc) ^(RS)−1   (4)

Herein, M_(sc) ^(RS) is the number of subcarriers for transmission ofDMRSs in each SC-FDMA symbol, as described above.

Embodiment 1

FIG. 5 is a diagram illustrating an exemplary embodiment of the presentinvention where an UE allocates DMRSs through 2 TX antennas.

Referring to FIG. 5, an UE transmitting signals through 2 TX antennasallocates DMRSs to the TX antennas by alternately allocating thepositions of the DMRSs in subcarrier group bands allocated to the TXantennas.

That is, all the DMRSs for the 2 TX antennas are consecutively allocatedat the subcarrier positions for the UE, but the DMRSs for each TXantenna are alternately allocated at the subcarrier positions without anoverlap therebetween.

A base sequence r _(u,v)(n) has orthogonality by using a CAZAC sequenceof a cyclic shift type. Thus, a Zadoff-Chu sequence, a kind of CAZACsequence, may be used as described above.

If the UE uses 2 TX antennas, a DMRS r_(u,v) ^((α))(n) is generated by abase sequence r _(u,v)(n) and a cyclic shift a as Equation (5) below.

r _(u,v) ^((α))(n)=e ^(jon) r _(u,v)(n), 0≦n≦M _(sc) ^(RS)   (5)

Herein, M_(sc) ^(RS) is the number of subcarriers for transmission ofDMRSs in each SC-FDMA symbol for each TX antenna.

In this embodiment, if the number of subcarriers allocated to eachSC-FDMA of the UE is N_(SC),

${M_{sc}^{RS} = \frac{N_{SC}}{2}},$

because each UE has 2 TX antennas

That is, the length M_(sc) ^(RS) of a sequence required for each TXantenna is equal to

$\frac{N_{SC}}{2},$

for the number N_(SC) of subcarriers allocated to each SC-FDMA.

Thus, DMRSs allocated to each TX antenna are expressed as Equation (6)below.

$\begin{matrix}{r_{{2k} + {{({k_{0} + q})}\mspace{11mu} \% \; 2}}^{q} = \{ \begin{matrix}{r_{u,v}^{\alpha}(k)} & {{k = 0},1,\ldots \mspace{14mu},{M_{sc}^{RS} - 1}} \\0 & {otherwise}\end{matrix} } & (6)\end{matrix}$

Herein, q is a TX antenna number in the UE, k is a subcarrier number foreach TX antenna, and k₀ is a position offset of the subcarrier, and(k₀+q)%2 denotes the remainder of the division of (k₀+q) by 2 (i.e., thenumber of TX antennas), that is, a modulus operation.

Embodiment 2

FIG. 6 is a diagram illustrating another exemplary embodiment where eachUE allocates DMRSs through 4 TX antennas.

Referring to FIG. 6, an UE transmitting signals through 4 TX antennasallocates DMRSs to the TX antennas by allocating a DMRS to each TXantenna only at one subcarrier position among the 4 consecutivesubcarrier positions.

That is, all the DMRSs for the 4 TX antennas are consecutively allocatedat the subcarrier positions for the UE, but the DMRSs for each TXantenna are alternately allocated at the subcarrier positions without anoverlap therebetween.

As described above, the length M_(sc) ^(RS) of a sequence required foreach TX antenna is equal to

$\frac{N_{SC}}{4},$

for the number N_(SC) of subcarriers allocated to each SC-FDMA withrespect to the 4 TX antennas.

Thus, DMRSs allocated to each TX antenna are expressed as Equation (7)below.

$\begin{matrix}{r_{{4k} + {{({k_{0} + q})}\mspace{11mu} \% \; 4}}^{q} = \{ \begin{matrix}{r_{u,v}^{\alpha}(k)} & {{k = 0},1,\ldots \mspace{14mu},{M_{sc}^{RS} - 1}} \\0 & {otherwise}\end{matrix} } & (7)\end{matrix}$

Herein, q is a TX antenna number in the UE, k is a subcarrier number foreach TX antenna, and k₀ is a position offset of the subcarrier, and(k₀+q)%4 denotes the remainder of the division of (k₀+q) by 4 (i.e., thenumber of TX antennas), that is, a modulus operation.

Embodiment 3

FIG. 7 is a diagram illustrating another exemplary embodiment where eachUE allocates DMRSs through P TX antennas.

For the number N_(SC) of subcarriers allocated to each SC-FDMA withrespect to the P TX antennas, the length M_(sc) ^(RS) of a sequencerequired for each TX antenna is equal to

$\frac{N_{SC}}{P}.$

Thus, for each UE transmitting MIMO SC-FDMA through P TX antennas, DMRSsallocated to each TX antenna are expressed as Equation (8) below.

$\begin{matrix}{r_{{Pk} + {{({k_{0} + q})}\mspace{11mu} \% \; P}}^{q} = \{ \begin{matrix}{r_{u,v}^{\alpha}(k)} & {{k = 0},1,\ldots \mspace{14mu},{M_{sc}^{RS} - 1}} \\0 & {otherwise}\end{matrix} } & (8)\end{matrix}$

Herein, q is a TX antenna number in the UE, k is a subcarrier number foreach TX antenna, and k₀ is a position offset of the subcarrier, and(k₀+q)%P denotes the remainder of the division of (k₀+q) by P (i.e., thenumber of TX antennas), that is, a modulus operation.

A number of exemplary embodiments have been described above.Nevertheless, it will be understood that various modifications may bemade. For example, suitable results may be achieved if the describedtechniques are performed in a different order and/or if components in adescribed system, architecture, device, or circuit are combined in adifferent manner and/or replaced or supplemented by other components ortheir equivalents. Accordingly, other implementations are within thescope of the following claims.

1. A method for allocating DeModulation Reference Signals (DMRSs),comprising: generating DMRSs; and allocating the DMRSs at consecutivesubcarrier positions with respect to all transmit (TX) antennas of eachUser Equipment (UE) and allocating the DMRSs at different subcarrierpositions with respect to each TX antenna of the UE.
 2. The method ofclaim 1, wherein the generating of DMRSs comprises cyclically shifting abase sequence as many as the number of subcarriers for each TX antenna.3. The method of claim 1, wherein the allocating of the DMRSs allocatesthe DMRSs to the respective TX antennas sequentially one by one.
 4. Themethod of claim 1, wherein the allocating of the DMRSs allocates theDMRSs to the respective TX antennas one by one in a subcarrier groupband where as many DMRSs as the number of the TX antennas of the UE areconsecutively allocated.
 5. An apparatus for allocating DeModulationReference Signals (DMRSs), comprising: a multiplexer multiplexing DMRSsby frequency division; and a subcarrier resource mapper allocating themultiplexed DMRSs with respect to transmit (TX) antennas of each UserEquipment (UE), wherein the subcarrier resource mapper allocates theDMRSs at different subcarrier positions with respect to each TX antennaof the UE and allocates the DMRSs at consecutive subcarrier positionswith respect to all the TX antennas of the UE.
 6. The apparatus of claim5, wherein the DMRSs are generated by cyclically shifting a basesequence as many as the number of subcarriers for each TX antenna. 7.The apparatus of claim 5, wherein the subcarrier resource mapperallocates the DMRSs to the respective TX antennas one by one in asubcarrier group band where as many DMRSs as the number of the TXantennas of the UE are consecutively allocated.
 8. The apparatus ofclaim 5, wherein the subcarrier resource mapper allocates the DMRSs tothe respective TX antennas sequentially one by one.